The fission rate and thus the heat generation in modern power reactors is usually controlled by the insertion and removal of control rods into or between fuel assemblies in the reactor core. Particularly in reactors of the pressurized-water type, cylindrical control rods are reciprocated within guide tubes which are part of the fuel assembly. The fuel assemblies are subjected to the flow of primary coolant in order to remove the heat generated in the fuel. When in the core, control rods are also heated through the nuclear transformation associated with their high neutron absorption rate, and so the control rods must also be cooled. Thus, a minimum of coolant must flow through the guide tube and over the rod at all times to control the temperature of the rod.
During power production, most of the control rods are maintained substantially withdrawn above the core. The lower tips of the rods, however, are not completely withdrawn from the guide tubes. Recent operating experience has shown that rodded fuel assemblies having been in an operating reactor for a period of time have significant damage on the inner walls of the guide tubes at precisely the elevation corresponding to the control rod tips.
Flow-induced periodic vibrations have been observed in arrangements having cylindrical rods eccentrically located in an annular diffuser. It is believed that two kinds of vortices interact as follows to produce the periodic driving force. When the rod is centered in the tube, a diffuser vortex ring forms around the rod (like a donut) in the diffuser region as the flow separates upon exiting the guide tube. If the rod becomes eccentric in the tube, axial vortices, also called secondary flows, originate near the control rod tip and travel along the rod before entering the diffuser region where vortex bursting occurs. The interaction in the diffuser region of the bursting axial vortices with the diffuser vortex ring produces an oscillating driving force on the control rod.
It has been proposed to provide a circumferential fence in the flow path downstream of the diffuser, or to provide strakes in the outer wall of the diffuser, starting at the diffuser mouth and extending longitudinally downstream of the diffuser. These solutions are not practical for use in nuclear reactors because the diffuser region can be very large.
Since nuclear reactor control rods often are not exactly centered in their guide tubes, the rod tip has a tendency to assume a rest position against the inner wall of the tube. Any significant tip vibration against the inside of the guide tube could damage the inner wall by perforating the guide tube. It has been found that, except for significantly reducing the mass flow rate in the guide tube, the above suggested remedies for reducing flow-induced vibration of a control rod are only marginally effective. If the flow rate is reduced sufficiently to eliminate rod vibration, then the rod is not adequately cooled.
Still another approach to eliminating control rod vibration is disclosed in Buettiker U.S. Pat. No. 4,313,796 issued Feb. 2, 1982. In this system the guide tube post has been modified to provide a polygonal cross section at the exit to the diffuser region. When due to random fluctuation, the rod assumes an eccentric position near the wall of the post, the polygonal post geometry provides a relatively large angle between the post wall and the surface of the rod near the wall. It has been found that if this angle is too small, strong secondary flows develop along the length of the control rod and interact with other vorticies in the diffuser region, to produce the vibration driving force. The polygonal cross section in the post modifies the secondary flow to reduce the intensity of the interaction with the diffuser vortices.
In nuclear reactor designs where a tubular extension is provided from the upper end of the guide tube, a more simple mechanical/hydraulic structure can be provided to overcome forces on the control rod which create its destructive vibration. Not only is the mechanical/hydraulic structure simple in construction, but it can be readily mounted as a retrofit in a nuclear reactor.